Welfare standards are on average higher in laying hens kept in cages than in free range flocks, according to a leading veterinary expert.

Enriched cages, which have replaced battery cages, are not ideal but produce better conditions than some free-range farms, said Prof Christine Nicol of Bristol University.

Not all free-range farms meet standards consumers expect, she added.

Other research contradicts this, said Humane Society International.

About 50% of UK hens are now housed in enriched cages. The cages, which contain more space for birds, including a perch and nest area, typically contain around 90 hens and are stacked on top of each other in tiers.

Commercial hen keeping

Battery cages were banned in Europe in 2012, but 'enriched' battery cages are legally permitted

Battery cages now have to provide 600cm squared useable space per bird, and facilities for perching, nesting and scratching

Each wire cage typically houses around 80 hens, with as many as 9 rows of cages stacked on top of each other.

In barn systems hens have freedom and space to move around within a building. Perches and nest boxes are provided

Most free-range hens on commercial farms live in buildings like the barn system, but have access to the outside through openings called 'popholes'. EU laws say free-range hens must always have access to an outside area with adequate vegetation during the daytime.

Researchers at Bristol University compared the welfare of laying hens kept in different farming conditions, including battery cages (which have been phased out in the EU), enriched (or furnished) cages, barn systems and free-range.

Professor of animal welfare, Christine Nicol, said enriched cages performed better on animal welfare measures than some free-range farms.

"I'm not saying it's great, there's still a lot of room for improvement but the birds have space, they've got a little perch, they've got things they can scratch on," she said.

"And the welfare measures that we take show that these birds had fewer fractures, they had much lower mortality, they had lower stress levels, they did less damaging pecking to each other than the birds on the smaller free-range systems."

She said the battery cage was completely unacceptable and was right to be banned.

"The furnished cage that's replaced it at the moment gives the best welfare outcome but it is also limited, it is not the best system that there could be at all, and there's very little that you can do to improve the furnished cage further.

In a furnished cage system unlike a barn or a free-range system the animal doesn't have the ability to express their natural behaviour in the way they need toChetana Mirle, Humane Society International

"And it would be nice to think that the free-range system currently gave birds the best welfare. The problem is that the management of free range systems in the UK at the moment is so variable, that although you get some brilliant farms, you also get some that are really really not good."

She said the challenge for the hen industry was realising the potential of the free-range system "so that they actually do what consumers think they do which is provide all hens with good welfare".

Prof Nicol said the study from 2010 had been supported by more recent research.

However, Chetana Mirle, director of farm animal welfare for Humane Society International, said other research had produced conflicting results.

She said proper management of hens was essential in any environment to stop troublesome behaviours such as hen pecking.

But she said there was not enough space in enriched cages to allow birds to perch adequately or run and jump.

"In a furnished cage system unlike a barn or a free-range system the animal doesn't have the ability to express their natural behaviour in the way they need to," she told BBC News.

"A number of behaviours that are natural to a hen are not possible in a furnished cage system - that's an important indicator to look at."

Robert Gooch of the British Free Range Egg Producers Association said the most important criterion in bird welfare was allowing birds to express their natural behaviour.

"There are many criteria of bird welfare," he said. "The leading one is that birds should be able to express their natural behaviour and hence why free range and organic systems have been developed so birds can do that."

He said birds in free-range environments were exposed to risks such as predation by foxes or could injure themselves flapping from perches, which could account for injuries found in research studies.